Submarine optical gain equalizer, submarine optical transmission line and its installation method

ABSTRACT

A submarine optical gain equalizer installed on a submarine optical transmission cable  5 . The submarine optical gain equalizer comprises a pressure housing  1 , in which an optical gain equalizer and n through-fibers are installed. Input and output ports of the optical gain equalizer and the through-fibers are formed in the outside of the pressure housing. The submarine optical gain equalizer equalizes levels of signal light having wavelengths different each other caused by optical fiber amplifiers. In general, one submarine optical gain equalizer is installed for 40 optical fiber amplifiers on a submarine optical cable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a continuation application of U.S. patentapplication Ser. No. 09/063,041, filed Apr. 21, 1998 now U.S. Pat. No.6,236,776, and the complete contents of that patent are hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Wavelength Division Multiplexing(WDM) submarine optical repeater system and particularly to a submarineoptical gain equalizer installed on a submarine optical transmissionline, and the submarine optical transmission line and its installationmethod.

2. Related Background Art

In the WDM optical transmission system, to transmit multi-wavelengthoptical signals over a long distance, it is necessary to install lightamplifiers at predetermined intervals on an optical transmission line soas to amplify signal light. The light amplifiers, however, have gaincharacteristics slightly different among wavelengths, which causes leveldifferences between optical signals having longer wavelengths andshorter wavelengths. These level differences are accumulated in eachlight amplifier. Accordingly, some lines are disabled from being usedfor communications in long-distance optical transmission lines.Therefore, in a shore optical transmission system over a transmissiondistance of several hundreds of kilometers or so, light source power ofsignal light having different wavelengths has been previously adjustedbased on the characteristics of the light amplifiers, for example.

In a submarine optical communication system, however, withmulti-wavelength light having an optical transmission line exceeding10,000 km, for example, a method of adjusting the light source power ofthe signal light does not function appropriately in some cases. In thesubmarine optical communication system including such a long-distanceoptical transmission line, there has been no proposal of any specificconfiguration of gain equalization for adjusting respective wavelengthlevels. It is also possible theoretically to achieve an equipment fordetecting respective wavelength levels automatically and for adjustingthe levels. It is presumed, however, that this kind of the equipment isextremely complicated and requires high cost.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a submarine opticalgain equalizer for compensating efficiently for gain differences betweenwavelengths of multi-wavelength optical signals transmitted on an actualsubmarine optical transmission line at low cost, the submarine opticaltransmission line and its installation method.

The submarine optical gain equalizer of the present invention comprisesa pressure housing and an optical gain equalizer installed in thepressure housing having input and output ports formed in the outside ofthe pressure housing. In the pressure housing, one optical gainequalizer can be installed. In addition, in the pressure housing, one ormore through-fibers can be installed in the pressure housing. Theoptical gain equalizer attenuates levels of optical signals havingspecific wavelengths by predetermined amounts. On the submarine opticaltransmission line of the present invention, one or more submarineoptical gain equalizers in the above are installed. An installationmethod of the submarine optical transmission line comprises detectingrespective wavelength levels of multi-wavelength optical signals inrespective optical fibers composing the submarine optical transmissionline and connecting submarine optical gain equalizers for compensatingfor the predetermined level differences between the detected wavelengthsin the detected positions of the optical fibers. This invention providesan efficient and low-cost compensation for the level differences inmulti-wavelength light transmitted through the long-distance submarineoptical transmission line.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionwhen taken with the accompanying drawings in which:

FIG. 1 is a diagram illustrating a configuration of a submarine gainequalizer according to the present invention;

FIGS. 2A and 2B are diagrams illustrating levels of a multi-wavelengthoptical signal before and after compensation;

FIG. 3 is a diagram illustrating a configuration in which two submarineoptical gain equalizers are installed on a submarine opticaltransmission line; and

FIGS. 4A and 4B are diagrams illustrating levels of multi-wavelengthoptical signals before compensation, and FIGS. 4C and 4D are thoseillustrating levels of multi-wavelength optical signals aftercompensation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown an embodiment of a configuration inwhich a submarine optical gain equalizer of the present invention isinstalled on a submarine cable 5. A pressure housing 1 contains anoptical gain equalizer 2 and n through-fibers 3. Input and output portsof the optical gain equalizer 2 and the through-fibers 3 are formed inthe outside of the pressure housing 1. n+1 optical fibers 7 in thesubmarine cable 5 are connected to the input and output ports of theoptical gain equalizer 2 and the through-fibers 3 at splice points 4. Inthe same manner as for a normal submarine optical repeater, the pressurehousing is a cylinder made of beryllium copper as a material having athickness of 1 to 2 cm and a diameter of 20 to 30 cm on a cross-section.

FIG. 2A illustrates a state of a level difference, adB between theshortest wavelength and the longest one in multi-wavelength opticalsignals having eight wavelengths in the optical fiber connected to theoptical gain equalizer 2. Under this condition, after themulti-wavelength optical signals are transmitted through the opticalgain equalizer 2, the optical signals having wavelengths other than theshortest one attenuate so as to be equalized to the level of theshortest one as shown in FIG. 2B. Other optical fibers are connected tothe through-fibers 3 even if there are significant level differencesbetween wavelengths of multi-wavelength optical signals transmittedthrough the optical fibers. The optical gain equalizer 2 is manufacturedwith its attenuation characteristics being calculated based onconditions such as wavelengths of signal light, the number of installedlight amplifiers and their characteristics, lengths of optical fibers,and a temperature of the sea bottom where the submarine cable is to beinstalled. The optical gain equalizer 2 is a known device, specificallyof a fiber grating type or of an etalon type configured so as to obtainattenuation amounts depending upon wavelengths.

Referring to FIG. 3, there is shown a configuration including twosubmarine optical gain equalizers 6 having attenuation characteristicsdifferent from each other on the submarine optical cable 5. In thesubmarine optical repeaters 8 shown in FIG. 3, an optical fiberamplifier and other devices are installed. If characteristics of eachlevel difference of multi-wavelength optical signals in a fiber 10 inFIG. 3 are as shown in FIG. 4A and characteristics of each leveldifference of multi-wavelength optical signals in a fiber 11 in FIG. 3are as shown in FIG. 4B, it is necessary to install submarine opticalgain equalizers having attenuation characteristics which meet thecharacteristics of the level differences of the multi-wavelength opticalsignals for these two fibers. Under these conditions, the opticalsignals transmitted through the submarine optical gain equalizers havesuch characteristics that the multi-wavelength signal light has the samelevel in both of the fibers as shown in FIGS. 4C and 4D.

The submarine optical gain equalizer 6 of the present invention isconnected to optical fibers in the submarine cable 5 when the leveldifference between the shortest wavelength and the longest one of themulti-wavelength signal light becomes 2 dB to 3 dB. In a long-distancesubmarine optical transmission line, light amplifiers are generallyinstalled at intervals of approx. 40 to 50 km. Giving an example of8-wavelength light having intervals of 0.8 nm and assuming a temperatureerror 2° C. and an input level error of an optical signal into eachlight amplifier 0.5 dB, a level difference 2 dB between the shortestwavelength and the longest one arises when the optical signal has passedthrough 40 light amplifiers. Therefore, it is preferable to install theabove submarine optical gain equalizers at intervals of approx. 1,600 kmon the submarine cable 5.

An installation method of the submarine optical gain equalizers 6 on thesubmarine cable 5 will be described below. First, based on theconditions such as the characteristics of the light amplifiers and thesea temperature in the submarine cable installation sea area asmentioned above, submarine optical gain equalizers having fixedattenuation characteristics of certain attenuation amounts forrespective wavelengths are manufactured. Also, submarine optical gainequalizers having attenuation characteristics values of respectivewavelengths slightly different from those of the above submarine opticalgain equalizers are prepared. Then, multi-wavelength light having thesame wavelength as for actual signal light is practically transmitted toeach optical fiber from an end office. At this point, it is checked thata level difference between the wavelengths becomes practically apredetermined value in a spot on the sea away from the end office by anestimated distance, and then a submarine optical gain equalizer 6 isconnected, the equalizer also having been previously designed so as tobe adapted to the level difference. If the level difference of themulti-wavelength light differs from the estimated one, a submarineoptical gain equalizer having other attentuation characteristics isused, this equalizer also having been prepared in advance instead of theabove submarine optical gain equalizer for the connection. In thismanner, an optimum submarine optical gain equalizer 6 is installed forrespective fibers.

The multi-wavelength optical signal used in the above embodiment has a1.55μ bandwidth with each wavelength having 0.8 nm to 1.0 nm intervals.

As set forth hereinabove, the present invention provides an optimumsubmarine optical gain equalizer for each optical fiber installed in arequired position so as to compensate efficiently and easily at low costfor level differences between wavelengths caused by characteristics oflight amplifiers or other conditions.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by the present invention is not limited to thosespecific embodiments. On the contrary, it is intended to include allalternatives, modifications, and equivalents as can be included withinthe spirit and scope of the following claims.

What is claimed is:
 1. An installation method of a submarine opticaltransmission line comprising the steps of: detecting each wavelengthlevel of multi-wavelength optical signals at a plurality of positions inrespective optical fibers comprising a submarine optical transmissionline; connecting submarine optical gain equalizers for compensating forpredetermined level differences between the detected wavelengths in thedetected positions on the optical fibers; and installing said opticalgain equalizers in a pressure housing having input and output ports forsaid optical fibers being formed on an outside of said pressure housing.2. An installation method of a submarine optical transmission lineaccording to claim 1, further comprising the steps of: previouslycalculating attenuation degrees of respective wavelengths of themulti-wavelength optical signals in the positions on said submarineoptical transmission line where said submarine optical equalizers areinstalled; preparing the submarine optical gain equalizers for adjustingthese attenuation degrees; and installing said submarine optical gainequalizers in said positions on said submarine optical transmissionline.
 3. An installation method of a submarine optical transmission lineaccording to claim 2, further comprising the steps of: preparing secondsubmarine optical gain equalizers having attenuation characteristicsdifferent from attenuation characteristics of said first mentionedsubmarine optical gain equalizers; checking level differences ofrespective wavelengths of the multi-wavelength optical signals in thepositions on said submarine optical transmission line where saidsubmarine optical equalizers are installed; determining if the checkedlevel differences exceed an estimated difference at each of thepositions and, if so, installing one of said second optical gainequalizers in place of one of the first mentioned optical gainequalizers at those positions where the checked level differences exceedthe estimated difference.
 4. An installation method of a submarineoptical transmission line according to claim 1, wherein said submarineoptical gain equalizers comprise an optical gain equalizer and one ormore through fibers, and wherein said step of connecting furthercomprises connecting said through fibers to the remaining fibers in saidsubmarine optical transmission line.